New technologies have improved the speed and accuracy of specimen preparation for materials such as ceramics, cermets, nitrides, borides and sintered carbides.

Microscopic examination is an important inspection technique widely used in the research, quality control and failure analysis of very hard materials. To properly reveal the microstructure of these very hard materials, it is critical to use the proper equipment (usually automated), along with suitable high-quality consumable products, such as cutting blades, mounting compounds, abrasives, lubricants and working surfaces with proven preparation methods.

Historical Perspective

About 35 years ago, evaluating failed sintered carbide (WC-Co) cutting tools was a slow process. Cutting these tools, some of which had hardnesses of around 1500 HV, was impossible with an ordinary abrasive cutoff saw.  However, they could be successfully sectioned using diamond-tipped wafering blades with a low-speed saw. This was a slow process at that time, as these were basically first-generation saws operating at no more than 300 rpm and low applied loads.

Grinding with silicon carbide (SiC) abrasives was possible, but the cutting rate was very low. Abrasive cutting theory has shown that the hardness of the abrasive must be at least 1.5 times that of the material to be abraded, and preferably more. SiC has a hardness of around 2200 HV, close to the minimum value. Polishing with several steps using finer and finer diamond abrasives (such as 9-, 6-, 3- and 1-µm diamond) on a napless cloth (usually nylon at that time) was a slow process but yielded good results.

A program in the late 1970s evaluated cermet cutting tools (a-Al₂O₃ with 30% TiC) for machining cast alloy steel rolls for hot rolling round bars. This mill was designed to roll bars to less than half the normal dimensional tolerance of hot-rolled steel bars. Carbide cutting tools wore too much during the machining of the mating roll passes, and the lathes did not have tool-wear compensation capability.

Sectioning of cermets was only possible with diamond-tipped wafering blades with a low-speed saw. Although this was a slow process, the cuts were of high quality (i.e., relatively damage-free and smooth). However, as the cermets were very hard, SiC paper could not be used to grind the specimens. At that time, the only harder abrasive products available for grinding were metal-bonded discs covered with diamond, available in several diamond sizes. The surface tension between the specimen and the disc surface was very high, which made them impossible to hand grind. A grinder with an automated specimen holder had to be used. The use of several diamond grinding discs with decreasing particle size, followed by polishing with finer diamond sizes on cloths, obtained good results.

Since the 1980s, a great deal of research has been devoted to developing ceramic materials and, more recently, nitrides. The only suitable abrasive for most of these hard materials is diamond. Cubic boron nitride (CBN) abrasive could be used to prepare some of the lower-hardness ceramics and nitrides, but little use has been made of CBN except on wafering blades for steels. Fortunately, substantial improvements have been made in the equipment used to prepare hard materials and in the abrasive products themselves. Specifically, the low-speed saw has been enhanced to increase its speed and capabilities.

Preparation Methods

The cutting of very hard materials still requires diamond abrasive blades. Though they are rather expensive, it is possible to buy large-diameter diamond abrasive blades for large saws. For sectioning rather large specimens, these blades are indispensible. If the part to be cut is less than approximately 1 in. (25 mm) in diameter, then less-expensive diamond-edged wafering blades and precision or low-speed saws may be used. These are more cost-effective but cannot be used to section large specimens. Wafering blades have not changed dramatically over the past 40 years, although they have gotten larger in diameter, from 3 to 8 in. (75 to 200 mm).

Today, a variety of fixed diamond products are available for grinding that make preparation of these materials rather simple. It is possible to grind specimens using rather coarse diamond abrasive on hard cloths, but this is not a good procedure. Basically, the coarse grinding procedure is performed with two types of products: rigid grinding discs where diamond slurries are applied, or fixed abrasive discs where the diamond is incorporated into the disc permanently.

Of these later discs, the diamond is attached by either metal deposition (e.g., electroless nickel) or with a resin bond. Resin-bonded diamond discs are less aggressive than the metal-bonded variety. Within these diamond discs there are still other options as to how the diamond is distributed on the disc surface. Fixed diamond discs generally feature diamond particle sizes from about 70 to 15 or 9 µm, although both coarser and finer discs are available.

Polishing with diamond particle sizes ≤ 9 µm is generally done using diamond paste or diamond slurries applied to hard woven cloths. Diamond particle sizes down to 1 µm and occasionally smaller (0.5 or 0.1 µm) are used on cloths. In some cases, these fine diamond sizes are the last step in the process. It is also possible to use oxide polishing compounds for the final step, even though their abrasives are not nearly as hard as diamond.

Table 1 shows one approach to preparing very hard oxides, carbides, cermets and nitrides. Specimens are cut with the precision saw using diamond-tipped wafering blades and are then mounted in a polymeric resin for ease of handling and identification. The method in Table 1 uses two resin-bonded diamond grinding discs (DGD) and then uses three polishing steps with diamond abrasive paste or slurry applied to hard woven cloths. The 9-µm step, and even the 3-µm step, can be replaced with DGDs of the same particle size, if desired.

Table 2 shows an alternative approach using a rigid grind disc (RGD). The process could be started with a coarser diamond abrasive if the initial grinding step must remove more material. The 3-µm step could also be conducted using a rigid grind disc. If this is done, the 1-µm step should be increased to 5 minutes. The use of any diamond on a RGD is more aggressive with lapping action that produces a dull surface; use of a cloth and diamond of the same size, on the other hand, produces a shiny, polished surface.  

Because of the difference in cutting action, if the first polishing step after use of the RGD is with 1-µm diamond, greater time will be required to obtain a properly polished surface quality. Final polishing with the oxide slurry could also be performed on a vibratory polisher. This is particularly worthwhile when preparing sintered carbide materials, regardless of the complexity of the carbides in these materials, as the interfaces between the binder and the carbides will be revealed much more clearly, permitting very high magnification examination. Etchants for sintered carbides have been developed and used for many years. Table 3 shows three of the most useful etchants for sintered carbides.

Preparation Examples

The preparation procedures described in Tables 1 and 2 have been used to prepare a variety of very hard materials. The following examples of microstructures are presented to illustrate the wide range of very hard materials that exist and the microstructural results that can be obtained with properly prepared specimens.

Figure 1 shows a typical cermet cutting tool consisting of 30% titanium carbide (TiC) in an a-alumina (Al₂O₃) matrix. Examination of the as-polished microstructure shows a very strong contrast between the dark a-alumina matrix and the bright, highly reflective titanium carbides. Figure 2 shows the microstructure of barium titanate (BaTiO₃), a ferroelectric ceramic, in the as-polished condition and after etching with Keller’s reagent.

Figures 3 and 4 illustrate the microstructure of a lead zirconium titanate (PZT)-Ag-Pd multilayer ceramic actuator. PZT [Pb(Zr_x, Ti_1-x)O₃] is a ferroelectric ceramic perovskite material with a strong piezoelectric effect (a voltage differential is created across its faces when compressed).

Figure 3 shows as-polished images of the actuator in bright-field illumination and in Nomarski differential interference contrast (DIC) illumination. DIC reveals a roughness of the PZT ceramic surface due to its grain structure, which is revealed in Figure 4 after etching with a solution consisting of 100 mL water, 10 mL HCl and three drops of HF.

Figure 5 shows an as-polished cubic boron nitride specimen (CBN), a very hard material that is an electrical insulator but conducts heat well.

Figure 6 shows as-polished microstructures of titanium diboride (TiB₂), a very hard ceramic with a hexagonal crystal structure. Note that polarized light reveals more of the microstructure than bright field.

Figure 7 shows the microstructure of a molybdenum disilicide (MoSi₂), a refractory ceramic in a SiC matrix. Polarized light, plus a sensitive tint filter, reveals the grain structure very well.

Best Practices

Metallographic examination of very hard, inert ceramics, cermets, nitrides, borides and carbides can be readily performed using modern equipment and consumable products. Due to these materials’ high hardness, diamond is the chief abrasive used.

Specimens can be prepared with a high degree of flatness and edge retention while avoiding damage to the material due to plucking or excessive fracturing. The metallographer should not overlook the use of polarized light or Nomarski DIC to examine specimens. While many of these materials may be rather difficult to etch to reveal the grain structure, solutions and procedures are available for most of these materials.

For more information regarding sample preparation for hard materials, contact Buehler Ltd., 41 Waukegan Rd., Lake Bluff, IL 60044; (847) 295-6500; fax (847) 295-7979; e-mail info@buehler.com; or visit www.buehler.com.

Links

• Buehler Ltd.